The present invention relates to a system for testing the networks of printed circuit boards without any physical contact therewith, and more particularly to a system and method of using an electron beam to test for shorts between and opens in conductor traces of a printed circuit board.
It is desirable in the manufacture of printed circuit boards to test for defects at an early stage of fabrication in order to minimize the costs of repairing such defects and to maximize the yield of operable devices. The traditional approach to testing such circuitry has been to use mechanical devices, such as pins or other mechanical probes, to contact points on the circuitry to be tested and to run a current therethrough. However, the small size and high density of modern printed circuit boards makes the use of such mechanical testing devices unfeasible. The networks of modern printed circuit boards are becoming so small and so compactly placed on the boards that using a mechanical probe is difficult, if not impossible. Moreover, as the networks become smaller, the risks associated with damaging the networks using mechanical probes increases. For these reasons, systems using electron beams to provide contactless testing have been developed. However, although such systems exist, these known systems suffer from a number of disadvantages.
U.S. Pat. No. 4,843,330 to Golladay et al. discloses an electron beam contactiess testing system which includes a conductive grid 48 placed above the surface of the specimen 36 to be tested . The 48 grid is negatively biased while the networks to be tested are charged in order to repel secondary electrons back toward the specimen and enhance charging of the conductive material. The bias is removed, or a positive bias is applied, during scanning of the specimen by a beam 12 so that secondary electrons can be collected by a detector. The testing system also includes a table 32 selectively movable in the X-Y directions perpendicular to the axis of beam 12 to position specimen 36 within the beam deflection field.
U.S. Pat. No. 5,602,489 to El-Kareh et al. discloses a method for testing the interconnect networks of a multichip module for opens and shorts. An electron beam 570 lands on a pad of an interconnect network located on a substrate 500. The electron beam 570 is used to interrogate the pad. An extract grid 550 located above the substrate is maintained at a positive potential. While the electron beam 570 interrogates the pad, the pad emits secondary electrons until such a point that the pad reaches a positive potential near that of the positive potential of the extract grid 550. The extract grid is then switched to a negative potential. The pad, still being interrogated by the electron beam 570, then collects secondary electrons until such a point that the pad reaches a negative potential near that of the negative potential of the extract grid 550. The test time, the length of time it takes for the pad to change from the positive potential to the negative potential, is measured and compared to a reference value. From this comparison it can be determined whether the interconnect network is defect-free, open, or shorted. The increase or decrease in emitted secondary electrons are evaluated by an electron detector 560, and the substrate 500 is moved relative to the electron beam 570 by an x-y stage 540.
U.S. Pat. No. 4,169,244 to Plows discloses a system for testing electronic networks. The system includes an electron gun 2 for production of an electron probe 1, a scanning control for probe 1, and a holder 16 capable of supporting a specimen 25 such that the probe 1 can impinge substantially normally on the specimen 25. Deflection coils 6 are used to direct electron probe 1 to varying locations on the specimen 25.
All of these prior art patents disclose devices which use a focused electron beam, such as that generated by a scanning electron microscope. Such a beam is capable of charging/reading a small area, for example an area of 1 inch by 1 inch. When a larger board, or a panel of small boards, is to be tested it is therefore necessary to move the board (as does the X-Y table of Golladay et al. and the x-y stage of El-Kareh et al.). Moving the board, however, can greatly increase the time necessary for testing, particularly when numerous points on the board must be tested.
Moreover, all prior are references disclose systems which can perform only top-to-top testing of board networks. If the network passes through the board, the network cannot be charged at a point on one side of the board and then tested at a point on the other side of the board. This is a serious disadvantage, as many modern circuit boards have networks which pass therethrough.
Furthermore, the systems disclosed in all prior art patents require that secondary electrons emitted by the networks being tested be collected by a discrete electron detector. Such systems are prone to error, however, as emitted electrons may not necessarily be deflected toward the detector, and may therefore not be detected.
A further disadvantage with respect to El-Kareh et al. is that the test time, that is, the length of time it takes for the pad to change from the positive potential to the negative potential, must be measured and compared to a reference value for every pad tested. Depending on the materials used and the size of the pads, this test time may be great, thereby greatly increasing the time necessary for testing a board.
What is desired, therefore, is a system for testing printed circuit boards which tests the networks of printed circuit boards without any physical contact therewith, which quickly tests numerous points on the boards, which can test large boards or panels of small boards without requiring movement of the boards, which tests networks passing through the boards, and which reliably detects the presence of emitted secondary electrons.
Accordingly, it is an object of the present invention to provide a system for testing printed circuit boards which employs electron beams to test the networks of printed circuit boards without any physical contact therewith.
Another object of the present invention is to provide a system for testing printed circuit boards which quickly tests numerous points on the boards.
A further object of the present invention is to provide a system for testing printed circuit boards which employs an electron beam such that large boards and panels of small boards can be tested without requiring movement of the boards.
Still another object of the present invention is to provide a system for testing printed circuit boards which tests networks passing through the boards.
Still a further object of the present invention is to provide a system for testing printed circuit boards which tests both sides of printed circuit boards using an electron beam on each side.
Yet a further object of the present invention is to provide a system for testing printed circuit boards which reliably detects the presence of emitted secondary electrons.
These and other objects of the present invention are achieved by provision of a system for testing for opens in and shorts between conductor traces on a surface of a printed circuit board. The system includes an electron gun assembly for generating an electron beam and an electron optics assembly for directing the electron beam to the conductor traces on the surface of the printed circuit board, which conductor traces are at a reference potential. A grid located proximate to and substantially parallel with the surface of the printed circuit board is placed at a first potential before the electron beam is directed to a first point on a conductor trace to charge the conductor trace to a first potential. The grid is then placed at a second potential, the second potential being between the reference voltage and the first potential, before the electron beam is directed to a second point on the conductor trace to cause emission of secondary electrons. Any secondary electrons that reach the grid are collected by the grid, and a CPU is used to determine whether or not an open or short exists depending on whether secondary electrons are collected by the grid as the electron beam is directed to various locations on the conductor traces. The position and intensity of the electron beam are controlled by a raster/vector generator.
The electron optics assembly includes a focus coil having static and dynamic windings to focus the electron beam anywhere across the surface of the printed circuit board, at least one beam alignment yoke to position the electron beam precisely along the magnetic axis of the focus coil, a deflection yoke to produce magnetic fields that precisely position the electron beam anywhere across the surface of the printed circuit board, and an astigmatism corrector to compensate for any residual astigmatism caused by the focus coil and for any deflection astigmatism caused by the deflection yoke. The electron optics assembly also includes a static focus control for controlling the static winding of the focus coil and a dynamic focus driver for controlling the dynamic windings of the focus coil to adjust the focus of the electron beam for changes in focal length as the electron beam is deflected over the surface of the printed circuit board. A dynamic astigmatism correction generator is provided for controlling the astigmatism corrector to correct for spot distortions as the electron beam is deflected away from a center of the printed circuit board, and a geometric correction generator is provided for controlling the deflection yoke to correct for positional distortions as the electron beam is deflected away from a center of the printed circuit board.
Preferably, the electron gun assembly is surrounded by a gun chamber, the electron optics assembly is surrounded by an electron optics chamber and the printed circuit board is surrounded by a test chamber, and the system includes a vacuum system. The vacuum system creates a vacuum in the test chamber, a vacuum in the electron optics chamber greater than the vacuum in the test chamber, and a vacuum in the gun chamber greater than the vacuum in the electron optics chamber.
Also preferably, the system utilizes a spot analyzer for adjusting focus, astigmatism and geometry of the electron beam, and for measuring the spot size of the electron beam. The spot analyzer includes a target plate having a plurality of polygonal apertures on the face. A plurality of sensors are positioned inside the target plate under the plurality of polygonal apertures. The signals from the sensors are indicative of the quality of an electron beam striking the polygonal aperture above each sensor. Beam control signals are provided by the raster/vector generator for directing the beam to strike the apertures above the sensors. Signal processing electronics are also provided for amplifying, filtering, and performing analog-to-digital conversion of the signals generated by the sensors and for generating processed signals, which can be used by computational software executing the CPU to adjust the electron optics assembly.
In another aspect, the target plate can contain a plurality of polygonal apertures on two opposing faces with the plurality of sensors designed to receive signals from the plurality of sensors on both faces. Such a configuration will permit aligning of two sets of electron optics in a two-sided, dual beam testing system.
Most preferably, each surface of the printed circuit board includes at least two fiducial marks thereon. Software executing on the CPU aligns the deflection axes of each of the electron optics assemblies with respect to each surface of the printed circuit board based upon secondary electron signals generated by the fiducial mark(s).
In another aspect, the present invention provides a system having two electron gun assemblies, two electron optics assemblies and two grids, one of each placed on each side of a printed circuit board having two faces so that traces on both sides of the board, or traces passing through the board, can be tested simultaneously.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.